This invention relates to composite materials that display negative elastic constants, in particular, although not necessarily solely, for use in materials for low frequency sound reflection, absorption or sound insulation.
A variety of materials exist and endeavour to insulate against sound transmission. Examples of such existing formulations include acoustic panels such as described in U.S. Pat. No. 5,564,241 in which an acoustic layer is provided in or bonded to a structural layer. Other materials such as that described in U.S. Pat. No. 5,512,715 are highly complex composite materials. In all cases, difficulty is found in providing a shield to low frequency sound waves. The greatest difficulty is that, in order to cause sound wave scattering or reflection, the size of the scatterers must be of a similar order to the wave length. As low frequency sound has a long wave length, it follows that shielding materials would have to be either very thick or very soft. Naturally, very thick materials are not suitable in many applications including the building industry where there may be limitations on the thickness which can be accommodated. Very soft materials allow a thinner layer to be used as the soft materials shorten the wavelength, however, the use is limited by the structural weakness of the soft materials.
It is normal that static elastic constants of materials are positive. This is a required condition of structural stability otherwise a spontaneous deformation allows the system to reach a lower elastic energy level. Instability is implied should the static elastic constants not be positive. However, should negative elastic constants be possible, at least at particular frequencies, sound waves at those frequencies would be unable to propagate in such a material. Therefore, a material capable of having negative effective elastic constants at low frequencies may allow a relatively thin portion of material to act as a filter or shield against a particular target frequency.
It is an object of the present invention to provide a composite material with negative elastic constants over at least a particular frequency ranges to provide a shield or insulation against such frequencies.
Accordingly, in a first aspect, the invention may be seen to consist in a composite material having a matrix material being a general structural material, zones of soft elastic material within the matrix and embedded zones of solid, relatively high density and high rigidity material within the soft elastic material.
Accordingly, in a second aspect, the invention consists in a method of making a composite material having negative elastic constants at finite frequencies comprising embedding solid particles of reasonably high density and high rigidity within regions of soft elastic material and said regions of soft elastic material being embedded within a matrix material being a suitable structural material.